1. Field of the Invention
The present invention generally pertains to electronic communications and is more specifically directed to global positioning systems and direction-finding antennas.
2. Description of the Prior Art
Global Positioning Systems (GPS) and direction finders are well known in the art and separately provide latitude, longitude and altitude information, and pointing direction, respectively. As used herein, pointing direction is synonymous with the terms bearing, heading, azimuth, bearing angle, and antenna azimuth pointing angle, and these terms are used interchangeably to indicate a direction such as that determined by a magnetic compass. GPS utilized a system of twenty-four satellites, each of which has a radio frequency (RF) transmitter. GPS receivers decode the satellites' RF signals to calculate the latitude, longitude, and altitude of a position on the earth's surface. The particular orbits assigned to the satellites ensure that a GPS receiver located anywhere on the earth's surface (or on a ship or aircraft) will be able to receive RF signals from at least four satellites twenty-four hours a day. In addition to the typical latitude, longitude and altitude data, some GPS systems provide some direction information in one of three ways.
First, a few GPS systems use an array of either 3 or 4 antennas spaced several feet apart to provide direction information. The phase difference of the signals arriving at each of the antenna pairs is used to derive direction information. Roll, pitch and yaw information is provided by these systems. Unfortunately, these systems require a large surface, are not man portable, and are relatively expensive.
A second type of GPS system providing direction information is such one as that described in U.S. Pat. No. 5,146,231 which uses two antennas. One antenna has a sharp null in its azimuthal pattern. This antenna can be turned until substantially no signal is received. At that point, the null is pointing at a particular satellite. Unfortunately, GPS systems do not always have a large dynamic range and while the antenna null may be deep, in practice, background noise sometimes fills in most of the null and adds error and ambiguity to the measurements.
Third, some single-antenna GPS systems derive azimuth information from a comparison of previous positions. As long as the system is moving, the azimuth pointing data can be fairly accurate. However, if the system is stationary, the azimuth data cannot be derived. In other words, this scheme allows a direction of travel to be determined, but is not useful for determining an arbitrary direction while the GPS system is stationary.
Thus, there is a need for an instrument which has both position and direction finding capabilities and is also operable while the instrument is stationary so that an arbitrary direction may be determined. It is to the provision of such an instrument that the present invention is primarily directed.